In measurements of this type, which are used for locating cable breaks and for measuring transmission losses along optical fibres or fibre connections, it is possible to use e.g. an optical time domain reflectometer (OTDR) which is capable of measuring the attenuation variation along an optical fibre. This is performed precisely by measuring the temporal course of the echo from a pulse which is launched into the fibre. The range of the OTDR instrument, i.e. the length of the fibre on which measurements may be performed, is determined by the dynamic range of the instrument, as, of course, the echo signal becomes the weaker, the longer the distance it has travelled in the cable. Present OTDR instruments can typically have a range of the order of 100-200 km.
However, the development of erbium doped fibre amplifiers (EDFA) has made it possible to use fibre connections in which the fibres may be considerably longer than mentioned above. Accordingly, there is a need for enhancing the dynamic range of OTDR instruments or other corresponding instruments which measure on the echo from a pulse which is launched into the fibre. One way of enhancing the dynamic range of the instrument and thereby its range is to increase the power of the pulse which is launched into the fibre. It has been proposed to achieve this by using an optical fibre amplifier, such as e.g. an erbium doped fibre amplifier, which is arranged between the instrument and the fibre. Typically, such an optical amplifier will be capable of amplifying the signal in both directions so as to achieve amplification both of the pulse launched into the fibre and of the echo signal returning to the instrument from the fibre. Of course, it is also possible to employ the optical amplifiers already present in the fibre connection.
However, it has been found that even though the reflected signal is amplified hereby, the curve produced by the instrument and describing the attenuation as a function of the distance from the instrument, decreases sharply at a specific distance, which may typically be about 50 km. Thus, in reality, the range of the instrument is not enhanced, but, on the contrary, is diminished by the coupling of an optical fibre.
The problem involved by the combination of the optical measuring instrument and the optical amplifier is that the optical amplifier itself generates noise which interferes with the measurement. Amplifier noise is generated by the amplified spontaneous emission (ASE) in the amplifier. While OTDR instruments are adapted to correct for the background radiation, i.e. the noise, during the intervals between the signal pulses, said background noise being determined by measuring the background radiation without pulse exposure, but this require that the noise behaves ideally and independently of pulse exposure. This is not the case, as the ASE noise from the optical amplifier depends on the signal passing through the amplifier. This means that the noise emitted by the amplifier decreases abruptly when a pulse is emitted, and then it slowly increases to a dwell level during the interval between the pulses. As it is the dwell level noise which is calibrated for in the OTDR instrument, the noise level used will thus be greater than the noise which is actually present when the measure is made, as this takes place immediately after the emission of the pulse. As a too large value is thus subtracted from the measured signal level to compensate for the noise, the reflections originating from the most remote parts of the fibre, in other words the weakest signals, will be negative. As logarithmic values of the reflected signal are calculated in the calculations performed in the OTDR instrument, there will be no results for the negative signals. An OTDR instrument in which the noise is sampled in the intervals between the transmitted optical pulses and averaged over a number of pulses is disclosed in EP-A-0 591 818.
WO-A-90/15980 discloses an arrangement for OTDR measurements on a fibre in which an optical amplifier is utilized to amplify optical pulses before they are launched into the fibre under test as described above. Also, this arrangement suffers from the mentioned drawbacks. Thus, the instrument can just show the fibre attenuation curve out to the point where the reflected signals become negative in the measurement.